In order to obtain high electrical performance in (Bi,Pb).sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.y ((Bi,Pb)SCCO-2223) high temperature superconducting composites, highly phase pure (Bi,Pb)SCCO-2223 with perfect texture and superior grain connectivity is desired. "Texture" refers to the degree of alignment of the oxide superconductor grains along the direction of current flow. "Connectivity" refers to the positional relationship of the oriented oxide superconductor grains, the nature of the grains boundaries and the presence of phase impurities disrupting intergrain connection.
Many parameters must be controlled and optimized during the fabrication and thermomechanical processing of (Bi,Pb)SCCO-2223 tapes in order to obtain satisfactory electrical properties. Electrical properties may be grouped into two categories: intragranular electrical properties and intergranular electrical properties. Intragranular electrical properties are those that are effected by changes within individual oxide superconductor grains. Critical transition temperatures (T.sub.c) is one electrical property which is predominantly intragrain. Critical current density (J.sub.c) and critical current retention (J.sub.ret) also has an intragrain component. Intergranular electrical properties are those which relate to the transport of a supercurrent across oxide superconducting grain boundaries and depend upon good intergrain connectivity. Critical current density (J.sub.c) and critical current retention in a magnetic field (J.sub.ret) have significant intergrain character.
In references too numerous to identify individually, the effects of powder composition, mechanical deformation, and heat treatment time, temperature and atmosphere on oxide superconductor formation have been studied. Not surprisingly, these studies have shown that heat treatment affects the rate of formation of the superconductor phase, the quality of the superconductor phase and the presence of secondary, non-superconducting phases. Thus, the heat treatment used in the formation of the oxide superconductor phase is important to the overall performance of the oxide superconductor composite.
Post-formation heat treatments have been investigated as a means for modifying the intragranular structure to boost performance properties of the oxide superconductor phase. Intragrain factors which affect electrical properties include the presence or absence of defects in the superconductor phase, and the phase purity of the superconductor phase and stoichiometric modifications thereof which may improve or degrade superconducting behavior. "Post-formation", as that term is used herein, means processing of the oxide superconductor after formation of the desired oxide superconductor phase from precursor oxide phases is substantially complete.
Typical post-processing heat treatments include annealing to alter the oxygen stoichiometry of the oxide superconductor phase, such as described by E. Ozdas and T. Firat in "Oxygenation Intercalation and Intergranular Coupling in the 110-K Bi.sub.1.7 Pb.sub.0.3 Sr.sub.1.8 Ca.sub.2 Cu.sub.2.8 O.sub.9.45+.delta. Superconductor" (Phys. Rev. B 48(13):9754-9762 (October, 1993)) and Idemoto et al. in "Oxygen Nonstoichiometry of 2223 Phase Bi--Pb--Sr--Ca--Cu--O System Superconducting Oxide" (Physica C 181:171-178 (1991)). They reported on the effect of heating (Bi,Pb)SCCO-2223 powders at temperatures from 500.degree. C. to 800.degree. C. and oxygen pressures of 0.2 to 10.sup.-3 atm. Idemoto et al. observed the formation of secondary phase Ca.sub.2 PbO.sub.4 and evaporation of PbO, while Ozdas and Firat reported inhomogeneities forming at oxide superconductor grain boundaries.
Um et al. (Jpn. J. Appl. Phys. 32: 3799-3803 (1993)) investigated the effect of a post-sintering anneal on (Bi,Pb)SCCO-2223 powders. They observe that T.sub.c is affected by the anneal temperature and oxygen pressure and found annealing at temperatures below 700.degree. C. and at oxygen partial pressure of 0.01 atm to provide optimized T.sub.c. Um et al. noted that the superconducting phase decomposes at temperatures higher than 700.degree. C. Wang et al. (Advances in Supercond. V (1992)) also found that post-annealing under vacuum at 790.degree. C. improved T.sub.c of (Bi,Pb)SCCO-2223 oxide superconductor powders.
These prior art references investigate the intragranular electrical properties of oxide superconductor powders and the authors are primarily interested in T.sub.c optimization. Oxide powders have no intergranular boundaries because of the random loose-packed nature of powder, and they provide no insight into the optimization of electrical transport properties (J.sub.c, J.sub.ret) of (Bi,Pb)SCCO-2223 superconductor current carriers, such as wires, tapes and the like.
Interestingly, the above-mentioned prior art noted the decomposition of the superconducting oxide phase and formation of secondary phases while optimizing intragranular electrical properties. Conventional wisdom would suggest that microstructures containing a non-superconducting secondary phase are undesirable because these particles disrupt local alignment of the BSCCO-2223 grains and decrease superconducting volume fraction in the composite. Thus, prior investigations have suggested that it is highly desirable to reduce the amount of secondary phases to as low a level as possible.
There has been little or no investigation of conditions which optimize the interconnectivity of (Bi,Pb)SCCO-2223 superconductor grains in a silver sheathed wire or which investigate its retention of critical current in the presence of a magnetic field. In the case of silver sheathed high temperature superconducting wires, good intergranular connectivity is critical to performance, yet processing is complicated by the need to move oxygen through the silver to the oxide superconductor. Observations made for oxide superconductor powders, which are an open system exposed directly to the furnace atmosphere and which systems do not include silver/oxygen interfaces, may not apply to silver sheathed tapes and the like.
The effects of cooling on the electrical properties of the oxide superconductor composite has been investigated by Lay et al. in "Post-Sintering Oxygen Pressure Effects on the Jc of BPSCC-Silver Clad Tapes" (Mat. Res. Symp. Proc. 275:651-661 (October, 1992). Lay et al. reported cooling in air at 1.degree. C./min resulted in a J.sub.c (77K, 0 T) increase over tapes cooled at 3.degree. C./min. Lay et al. also noted that holding the (Bi,Pb)SCCO-2223 samples at temperatures of 810.degree. C. or 780.degree. C. in reducing atmospheres improved J.sub.c.
While critical current (I.sub.c) and critical current density (J.sub.c) in self-fields may be useful indications of the quality of an oxide superconductor composite, an important performance parameter for in-field operations of oxide superconducting devices is their ability to retain their superconducting transport properties in the presence of a magnetic field. Many applications using oxide superconducting wires must be accomplished in the presence of its own induced magnetic field or in applied field ranging from 0.01 T to 100 T. Superconducting properties degrade dramatically in even relatively weak fields. Oxide superconductors show their most dramatic loss in critical current capacity perpendicular to the ab plane. Parallel to the ab plane, capacity loss is only a few percent. For example, weakly linked yttrium-barium-copper oxide superconductor (YBCO) exhibits a ten-fold drop in J.sub.c in magnetic field strengths of 0.01 T (B .perp. oxide superconductor tape plane). Conventionally processed BSCCO-2223 loses the majority of its critical current capacity in a 0.1 T field (77 K, .perp. tape plane). Even a few percent increase in critical current retention would have a dramatic effect on wire performance.
Thus, there remains a need to optimize the intergrain connectivity of high temperature superconducting wires and tapes so as to improve current carrying performance. Preferably, processing of an oxide superconductor wire or tape would enhance the intragranular properties of the conductor without detriment to the intergranular transport properties of the conductor. Due to secondary phase formation under conditions which optimize intragranular electrical properties, it is desirable to process the superconductor in a manner which minimizes the formation and/or detrimental effect of secondary phases on intergrain connectivity.
It is the object of the present invention to provide an oxide superconductor article with improved critical current retention and/or improved critical current density in the presence of a magnetic field.
It is a further object of the present invention to provide a method of treating the oxide superconductor composition to improve critical current retention and/or critical current density.
It is yet a further object of the present invention to increase flux pinning sites and/or intragranular coupling in a (Bi,Pb)SCCO-2223 oxide superconductor composite.
It is yet a further object of the invention to improve grain interconnectivity by reducing formation and/or the detrimental effect of secondary phase formation.
It is yet a further object of the present invention to provide a method form obtaining optimal intragranular properties of an oxide superconductor wire or tape, while minimizing the detrimental effects to intergranular transport properties.
These and other objection of the invention are accomplished by the invention as set forth hereinbelow.